The present invention relates to an endless belt having a power transmitting surface exhibiting high wear resistance, and more particularly to endless toothed belts having a fabric cover element intimately positioned along at least the outer surface of the tooth and land portions of the belt, and having a lubricating agent present within at least a portion of the fabric cover element, and optionally including a lubricating agent on at least a portion of the outer surface thereof, as well as to a method for producing such belts.
Endless belts, including V-belts, V-ribbed belts, and flat belting, as well as toothed belts such as synchronous- or timing belts and the like, are used in a variety of applications. Examples of power transmission belts, including toothed or synchronous belts, V-belts, and V-ribbed belts are disclosed in U.S. Pat. Nos. 3,138,962; 3,200,180; 4,330,287; and 4,332,576. Examples of methods for producing such belts are disclosed in U.S. Pat. No. 3,200,180 as indicated above and U.S. Pat. Nos. 3,772,929 and 4,066,732. These patent references are merely examples of various types of power transmission belts and known formation techniques thereof.
Toothed belts, generally comprising an elastomeric body portion, an essentially non-extensible reinforcing member and a plurality of driving teeth extending along the underside of the belt at a predetermined pitch, are put to particularly good use in high temperature, high speed and/or high load environments, including various industrial and automotive drive systems. In automotive applications, there is a growing demand for toothed belts that can perform successfully under increasingly high loads and at average operating temperatures of about 120° C. Operating temperature requirements for such applications are expected to reach 150° C. or greater in the near future.
Under such high load, high temperature and/or high-speed conditions, it is common for the teeth of endless toothed belts to deteriorate; the severe shearing stresses on the teeth often result in crack generation and tooth loss. A wear-resistant fabric cover element has been used over the tooth and land portions of such belts to shield the elastomeric teeth from such stresses. This modification alone however has not proved completely satisfactory in some particularly demanding applications. Upon extended high load or high-speed operation, such fabric covers tend to wear away, resulting in dimensional changes and/or premature belt failure. Moreover, there is a tendency in such constructions for the underlying belt elastomer to migrate through the weave of the fabric cover during the curing process and/or upon operation, and/or for the fabric cover to wear away with prolonged use, and to thus present some portion of the underlying belt elastomer at the belt's power transmitting surface. The presence of this relatively high coefficient of friction material at the belt's power transmitting surface results in high noise and frictional heat generation at the belt-sprocket interface upon operation of the belt. Noise generation is viewed as highly undesirable, and frictional heat generation and heat build-up reduce the life of the belt.
One proposed solution to the noise generation and/or frictional heat build up problems common in conventional belt operation has been to reduce the effective coefficient of friction of the power-transmitting surface of the belt. One such approach involves isolating or removing as much of the elastomer as possible from near the surface of the belt where that surface comes in contact with sprocket teeth. Such an approach is taken for example in U.S. Pat. No. 3,964,328, wherein the outer surface of a wear-resistant fabric covering is kept free of belt elastomer by the presence of a bonded layer of elastomer-impervious material adhered to such outer surface.
A second approach has been to incorporate a relatively pure polytetrafluoroethylene (PTFE) layer over the wear-resistant fabric cover element to decrease the effective coefficient of friction of the driving surface of the belt.
A third approach, disclosed in EP 1,052,425, directed moreover to improving the resistance of a belt's fiber cloth layer to wear, has involved applying a mixture including a resin adhesive ingredient, a rubber ingredient and a fluorine resin powder lubricant to both an exposed surface of such fiber layer and also to a second, opposed surface thereof, with fluorine resin powder being present at both the first and second fiber surfaces and between the fibers in the fiber layer between the first and second surfaces.
None of these approaches to the problems of abrasion or wear, noise and/or frictional heat generation in endless belt constructions is believed to be completely satisfactory, particularly in very high load applications. Where the belt surface remains free of belt elastomer by means of a relatively poor abrasion-resistant or low temperature laminate coating on the outer surface of a fabric cover element, high load or high temperature operation generally results in flaking off or melting of the coating, resulting in undesirable dimensional changes to the belt and poor tooth-sprocket fit, and hence increased belt noise. Moreover, as the coating layer diminishes, the fabric cover becomes exposed to the sprocket, ultimately leading to deterioration of such layer and exposure of the belt elastomer.
A substantially pure PTFE layer incorporated on the surface of a wear-resistant fabric cover element, while producing a reduced coefficient of friction at the driving surface similarly exhibits very poor wear resistance, and thus would likely wear off of the belt with use, again leaving the wear-resistant fabric layer exposed and presenting the concomitant problems associated therewith.
The incorporation of a mixture containing each of a rubber ingredient, a resin adhesive ingredient and a fluorine resin powder lubricant on both surfaces of and within a fiber layer of a power transmission belt is believed to be similarly insufficient. Application of such mixture onto a fiber layer surface according to the teachings of EP 1,052,425 is believed to result in deposition of a limited amount of actual lubricating specie at the power transmitting surface of the belt and within the fiber layer itself, since the lubricant is maintained within the belt composite structure in a space-occupying rubber/resin adhesive mixture. Moreover it is believed that the presence of the rubber and resin adhesive ingredients in the disclosed mixture would generally prevent lubricating specie within the fiber layer from reaching the power transmitting surface in order to contribute to the wear resistance thereof. In addition, since the fluorine resin powder lubricant is apparently dispersed homogenously in the rubber/resin ingredient binder and thus generally the same amount of the lubricating species would be found at the fiber layer-belt elastomer interface as at the opposite, power transmitting surface of the belt, it is anticipated that an undesirably high concentration of lubricating species may exist at or near the fiber layer-belt elastomer interface, having a potentially negative impact on adhesion of the fabric layer to the underlying belt body elastomer.
Thus, known endless belt constructions or processes for their manufacture have not effectively addressed the combined problems of belt noise, frictional heat generation, dimensional instability and durability.